JP5-73205A1 discloses an invention relating to a user interface using a visual line. According to the invention, a position on a display screen viewed by a user is detected, and a cursor is displayed at the detected position. Further, a position of a cursor is detected as point A when a button is pressed, another position of the cursor is detected as point B when the cursor is moved from point A and the button is pressed again, and an area of a circle, which is centered at point A and has a radius of a distance between point A and point B, is enlarged.
In a computer device having a touch screen, such as a tablet terminal, a user is able to enlarge or reduce an image by performing a pinch-close or pinch-open operation on the touch screen. However, in a situation where a user holds a tablet terminal with one hand, and the other hand is not free, s/he is not able to touch the touch screen with a finger other than a thumb, whereby it is difficult to perform an enlarging or reducing operation. In this situation, if the invention described in JP5-73205A1 is employed, the user is able to enlarge an image in the tablet terminal since pressing a button can be performed using one finger. However, it is necessary that such a button be available when the invention of JP5-73205A1 is employed. When a user holds a tablet terminal with one hand, s/he may hold it upright or horizontally; accordingly, a button is not necessarily positioned near a finger of the hand holding the tablet terminal, whereby the button cannot be pressed by a finger of the one hand holding the tablet to enlarge an image.